1. Field of the Invention
This invention relates to controlled state conversions between magnetic bubble domains having common winding numbers, and more specifically to controlled state conversion between bubble domains having winding number S=1.
2. Description of the Prior Art
In bubble domain devices, coding of information is done in a variety of ways. While the most common is binary coding using the presence and absence of domains to represent information, other schemes, such as the bubble lattice file, use coding where all bit positions are filled with bubble domains. In the lattice file, information is encoded by means of the winding or revolution numbers of the bubbles. The winding number is the number of rotations made by the wall moment at a point P when P makes one circuit around the periphery of the bubble. Examples of this type of coding and a procedure for producing bubble domains having winding numbers S=0 and S=1 has been described by Ta-Lin Hsu, AIP Conference Proceedings 24, 624 (1975). Generally, capping layers are used together with in-plane magnetic fields to provide stable bubble domain states having different winding numbers.
Detection of domains in a device such as the bubble lattice file is generally achieved by using a gradient deflection technique wherein bubbles having different winding numbers are deflected by different angles in a gradient magnetic field. This type of motion is described in U.S. Pat. No. 3,890,605 which describes bubble devices using coding in terms of the winding number of the domain.
In Applicants' copending application Ser. No. 659,880, filed Feb. 20, 1976, an "automotion" technique of distinguishing two wall states having the same winding number was described. For example, two automotion states were found for bubbles having a pair of unwinding vertical Bloch lines. These bubble domains were designated as .sigma..sub.+ domains and .sigma..sub.- domains and differed only in the combinations of magnetic polarities and local winding of their Bloch line pairs. In that copending application, the possibility of state conversion was described if the magnetic fields used to move the bubbles were too large or too small, depending upon a particular situation. It was observed that state conversion would occur when Bloch Lines were made to move into one another, collide and then annihilate. However, controlled generation of selected domain states was not taught in that application. The conversion margin plots were used to describe the margins for movement of bubbles by automotion. If there were state conversion, they would be accidental and the resulting state would not be a state selected in advance. Rather, the resulting state would be random and not well defined.
In contrast with that, the present application is directed to a technique for providing controlled state conversions between bubble domains where selected states can be produced. Additionally, the states which are produced are those having the same winding numbers so that the present invention is directed to controlled state conversions between domains having the same winding numbers.
Bubble domains having no vertical Bloch lines (unichiral domains) have the same winding number as bubble domains having a single pair of unwinding Bloch lines. Unichiral bubbles are desirable for representation of information since the absence of Bloch lines therein avoids state destabilization through nucleation of Bloch points, a process generally described by J. C. Slonczewski, in the AIP Conference Proceedings 24, 613 (1975). However, it has not been possible to easily identify the right-handed and left-handed unichiral states.
Accordingly, the present invention describes techniques for converting unichiral bubbles into .sigma. bubbles having the same winding number. Thus, devices using unichiral bubbles are now more practical, since the detectors need only provide state conversion from a unichiral bubble to a .sigma. bubble which is readily detectable using the principle of automotion. Thus, a device using unichiral bubbles, such as a bubble lattice file, would utilize a two-stage detector, where the first stage converts the unichiral bubble to a .sigma. bubble, while the second stage detects the resulting .sigma. bubble. Afterward, this .sigma. bubble or a newly nucleated one can be reconverted to its original unichiral state and then returned to storage.
Whereas the prior art used gradient techniques for providing S=0 and S=1 bubble domains, the present invention provides selected state conversions between bubble domains having the same winding number. Further, homogeneous magnetic fields having no spatial gradients are used.
Accordingly, it is a primary object of the present invention to provide controlled state conversion among bubble domains having the same winding number.
It is another object of the present invention to provide controlled state conversions between selected bubble domains having the same winding number using homogeneous magnetic fields.
It is another object to provide a technique for switching at will among domains having S=1 state using spatially uniform external magnetic fields.
It is a further object to provide a method and means for controllably providing state conversions between different bubble domain states having the same winding number, without the need for capping layers on the magnetic bubble domain material.
It is a still further object of this invention to provide techniques for switching between bubble domain states having the same winding number without requiring magnetic field gradients.